Conditional access applications, such as Pay-TV applications, use receivers for securely processing entitlements and storing decryption keys. Keys are typically organized in a key management structure with several layers. Each layer is used to deliver keys at a lower level layer. Keys are renewed to control access to the encrypted content. Keys for decrypting Pay-TV content are known as control words CW.
An entitlement message from a head-end system that provides a receiver with a new key value is called a positive entitlement. Positive entitlements, e.g. entitlement control messages (ECM), are typically secured and disadvantageously can introduce a significant bandwidth overhead. A negative entitlement is an entitlement message, e.g. an entitlement management message (EMM), that instructs a receiver to revoke and no longer use a particular key, resulting in the receiver being unable to decrypt Pay-TV content. Conditional access systems typically use a mixture of positive and negative entitlements. If the receiver blocks or removes the negative entitlement, it disadvantageously becomes possible for the receiver to use a non-authorized key for decrypting content.
Traditionally, Pay-TV implementations rely on hardware tamper resistance to protect the storage of cryptographic keys and to ensure the unmodified processing of entitlement messages. Examples of tamper resistant hardware are smart cards and secure computing chip devices embedded in Digital TV receivers. The increasing advances in chip manufacturing and the associated cost reductions makes it desirable for Pay-TV implementations to remove such special hardware components.
Pay-TV solutions are known that do not require specific tamper resistant chips. Such solution use software tamper resistance to protect the key storage and entitlement processing steps in a Digital TV receiver. Software tamper resistance technology is used in DRM systems for PCs, Mobile Telephones and IPTV devices.